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Dinosaurs being a bunch of giants is a myth, most were smaller than humans and the same size as animals today. Birds are actually classified as theropoda dinosaurs and they aren't that big are they [well...they are mostly avian].
Every once in a while there was a giant but those giants were not bigger than mammalian giants. African Bush elephant was about as big as any other dinosaur, the only exception are sauropods, those were the only giants that stood out in comparison to any other land animal in history.
Most of Sauropod species were actually not bigger than mammal giants but the few that were are a little mysterious, we don't know why they were so big but oxygen levels is likely not a reason. Some theories with strong basis are a very advanced respiratory system, which birds have [superior to mammals], another good theory is rich vegetation habitat combined with sauropods giving up on chewing. Rocks were found in sauropod abdominal area, it is theorized rocks in their stomach would grind food, thus the giants could eat more and not waste time chewing thus they got bigger [more food, more caloric intake, bigger size].
I've also heard theories that they were both endo and ectothermic. Sad truth is we will likely never know how they got so big. All non-bird dinosaur dna is long gone and you can't ''reverse engineer'' dna like some think, and even in a magical land where you could, the fact is theropods [birds] are cousins with sauropods, not children/parents.

Music got it right. They would roughly be the same size. Dinosaurs were obviously endothermic based on numerous bits of evidence and are closely related to birds. Birds are endotherms as well. Obviously elephants don't depend on their size to maintain their body temperature. A hundred years ago, scientists thought of dinosaurs as slow lizard like reptiles. Since then, due to very good scientists like Ostrom and Robert Bakker, nearly all experts accept that dinosaurs were warm blooded like birds. Dinosaurs might have to breathe slightly harder but that is about it. We still have 20 percent O2 in the atmosphere.

Dinosaurs weren't large because of high O2 concentration. That may be true of certain insects such as the very large dragonflies but not dinosaurs.
Insects are limited in size by the oxygen because of the way they breath (no lungs). Those limits don't apply to animals like birds, mammals, and dinosaurs.

I would also point out that there is no correlation with dinosaur size and temperature. In one instance the argument is they cook themselves if too large and the next they only grew big during warm times. That is ridiculous on its face.

would depend on how they were recreated. If from magid 100 mil year DNA, they'd be full size, but if enough of them lived for a thousand years, evolution would make them smaller. (male elephants are evolving with smaller tusks 'cause humans are wiping out the ones with genes for bigger tusks. I'd think that smaller dino's would survive to reproduce better than big ones.

They would be smaller today because of the cool climate, not because of oxygen levels. Whoever told you that dinosaurs were large because of high oxygen level does not know what he/she is talking about. During the age of dinosaurs, the climate was very warm because carbon dioxide (a greenhouse gas) levels were high in the atmosphere, and the warm climate was responsible for their large size. Dinosaurs are known as giganto-therms to physiologists because they rely on their size (and corresponding small body surface) to maintain a relatively constant and high body temperature even though they do not generate their own body heat. Physiologists point out that if dinosaurs were endotherms (animals that generate their own body heat), they would cook themselves from the inside out. A high level of atmospheric oxygen would have lowered the temperature, making it impossible for dinosaurs to get large, and a high level of atmospheric oxygen is not needed if an animal does not burn energy to stay warm. Reptiles and amphibians only use about 1/20 as much oxygen as mammals and birds. High levels of oxygen would have done them no good since they get all the oxygen they need already.

Dinosaurs are unlikely to be endotherms also because they have growth rings in their long bones. Further, when the climate cooled drastically (about as cool as it is today) at the end of the Jurassic, lots of large dinsoaurs, including many sauropods, the theropod Allosaurus and the ornithischian dinosaur Stegosaurus, became extinct. The smallest known dinosaurs, such as Compsognathus and Sinosauropteryx, also happened to live during the late Jurassic and early Cretaceous. Therefore if dinosaurs were still alive, they would have shrunk to the size they were during the late Jurassic to early Cretaceous. Dinosaurs only evolved large size again towards the end of the Cretaceous, when the climate have warmed back up to levels seen in the middle Jurassic.

Genetics play a bigger roll on size than anything else. By the time out technology could recreate dinosaurs, we could do it from scratch complete with all the adaptations to survive in our present biosphere. You would be disappointed as they would be nothing like the dinosaurs depicted in museums or books, more like animals we have today.
Consider, 70 million years ago, animals had adapted to a much thicker atmosphere, high humidity, and a warm stable temperature. How animals retained or reduced body temperature was a lot different. In studying plant fossils found in polar regions show the same as fossils found elsewhere showing even the poles were like a nominal 85 degrees day and night.
This sort of vegetation also needed a very high humidity to live.
The mass extinction event some 64 million years ago changed all that. What managed to survive are the ancestors of today's life and after evolutionary adaptation, what we have today, including man and our current biosphere.